Pancreatic cancer (PCa) is the 4th deadliest cancer in the US, with median life expectancy of less than a year. PCa does not respond well to conventional chemotherapies as the disease has progressed to late stages by the time of diagnosis. Gemcitabine (Gem) is commonly used as frontline treatment, but only extends patient survival by several months over other single agent or combination therapies. The combination of Gem + nab- paclitaxel was recently shown to extend survival by ~2 months over Gem alone. The FOLFIRINOX regimen is a formulation of multiple drugs that has shown promise by doubling (~4-6 months) survival of pancreatic cancer patients over gemcitabine alone. However, high toxicity associated with this treatment may limit the patients that can tolerate this regimen. Understanding and targeting the molecular mechanism responsible for chemoresistance is a rational approach to improving treatment outcomes. Inhibiting such pathways should allow existing drugs to kill more tumor cells, and thus extend patient survival. Importantly, chemosensitizers may reduce the effective dose of chemo and thus also reduce toxic side effects. Cancer cells rely critically on survival pathways that allow them to tolerate a broad range of extrinsic and intrinsic stresses that would normally block cell growth. Drugs such as gemcitabine, paclitaxel, and FOLFIRINOX inhibit DNA replication and chromosome segregation, two processes that are already destabilized in cancer cells. The survival pathways that have evolved to allow cancer cells to tolerate genome instability are likely responsible for allowing cells to survive treatment with these drugs. We conducted a genome-wide siRNA screen to identify genes that are critically important for the survival of pancreatic cancer cells treated with gemcitabine. This effort revealed an unexpected role for the Vitamin D receptor (VDR) transcription factor in dictating cell survival in response to drugs. VDR is activated by 1,25-OH vitamin D3 and is best known for regulating genes important for calcium homeostasis and bone mineralization. We have identified a new role in promoting DNA repair that is crucial for gemcitabine survival by pancreatic cancer cells. Our combined molecular, pharmacological and cell biological data strongly suggest that VDR is essential for pancreatic cancer cells to survive gemcitabine treatment. We believe that this reflects evolutionary adaptation of the importance of VDR in protecting skin from the harmful yet necessary benefits of UV in the production of Vitamin D. We propose in this R21 application to address how VDR is used by pancreatic cancer cells to survive gemcitabine and other chemotherapeutic agents. We will also conduct in vivo studies that test the efficacy of an anti-fungal drug in enhancing Gem killing of a PCa patient derived tumor xenograft, as well as a mouse model for the disease. This is an exciting R21 proposal because of its high translational potential and novel insights into how PCa cells survive drug treatment.